1. Field of the Invention
The device of this invention resides in the field of distribution of audio programs and telephone communication between a number of locations utilizing a time sharing system.
2. Description of the Prior Art
The device of this invention is a new and improved system relating to the distribution of selected audio programs and telephone communication to various locations such as is utilized in schools, offices, nursing homes and other establishments where such communication systems are necessary. The present art is clearly illustrated in school classroom situations where there is a loudspeaker located within each classroom through which the school administrator can make daily or random announcements to each room independently while sending other programs or announcements to other rooms at the same time. In many instances music is played into various rooms and the administrator has a plurality of channels available through which different audio programs can be sent to individual rooms. Further included within these communication systems is a telephone network by which an administrator can buzz a room and an instructor or other person will lift a hand set and be able to speak back and forth to the caller. These audio telephone distribution systems have been in use for many years and employ electromechanical and other well-known art in their structure. The systems are comprised of a number of audio sources such as AM and FM tuners, tape decks, and microphones for vocal communication. These audio sources are connected to a series of audio power amplifiers which may be selectively connected to the loudspeaker station located in a room by means of a bank of multiple position switches which complete the contact to that loudspeaker station. Most often a two-wire balanced system with two-pole switch is used. The power amplifiers utilized in this older system customarily employ an output transformer delivering a 70 - 25 volt RMS output maxium. Therefore a stepdown transformer is usually required at the loudspeaker station within the classroom. Volume control is achieved by utilizing different voltage taps on the stepdown transformer. Utilized with the loudspeaker system is a basic telephone network capable of multiple simultaneous conversations. Each simultaneous conversation requires a common wiring buss. To interconnect two telephones for a conversation, contact must be closed to the same link. Therefore, in order to have more than one simultaneous conversation, additional busses must be available for each simultaneous conversation, so that this older system has the number of possible simultaneous conversations limited by the number of busses within the network. The switches within such a telephone system are usually very low impedance contact relays, electro-mechanical devices or solid state approximation of relays. Closure of the relays, decoding of dial information and associated functions are usually performed by a control unit which in older units is mechanical and in newer units is of a solid state construction. The obvious disadvantage of the above system is that the amount of hardware required increases in proportion to the number of telephone links. As a rule the number of relay points equals the number of telephone stations times the number of links. A further disadvantage is that each telephone station requires its own set of contact wires for each link which increases both the cost of the control unit and the difficulty of installation. In the present art, when inteconnecting an audio distribution system with a telephone system, relays or mechanical devices must be employed to disconnect the loudspeaker station from its normal program connection and connect it to the appropriate telephone link. A disadvantage to this interfacing of a telephone system with a loudspeaker address system is that means for an individual at the loudspeaker station to communicate back to the originating telephone unit are limited. This disadvantage is partially solved by utilizing a talk-listen switch of either manual or voice activated types which employs the loudspeaker alternatively between duty as a loudspeaker and microphone.
There is a significant economic factor involved with the system utilizing the present art since the audio power signals are being switched many times. Expensive low impedance devices such as relays must be employed to perform all switching functions and there must be at least one set of switching devices for each station utilized within the telephone and loudspeaker stations and multiple switches are required if multiple simultaneous conversations are to take place. In recent years alternative systems have been designed wherein a minimum amount of hardware is utilized, but the hardware is "time shared" between several communications paths. The result is that one set of hardware is employed to handle all communications and audio programs as opposed to having a separate set of hardware available for each conversation. A time sharing system such as described above may include a master control unit which receives a series of audio program inputs which are sampled at discrete intervals of time and transmitted in digital format with address information including the intended receiver of the audio signal. At each receiving station is a local control unit which decodes the address information and, if it perceives that it is the address to which the information is directed, it will accept the digitized audio information. This digitized audio information is then reconstructed into its original analog format and amplified to a suitable power level. The local control units may also contain elements capable of formatting and transmitting audio call back information from the loudspeaker station. This system can be expanded to include telephone-serviced stations and provide other functions associated with telephone/audio distribution systems. There are several problems associated with these communication systems. Firstly, the sampled data audio system requires a theoretical minimum sampling rate of 6,000 samples per second per receiving station. With the inclusion of time allocated to control functions, systems requiring even a relatively small number of stations have operating rates approaching several megahertz. This high data rate presents problems in the transmission lines and in the design of the various control units. If the rate of sampling is lower than 6,000 samples/second, the sound quality is poor. These high sampling rates automatically prohibit the use of mechanical switching means, and solid state means must be employed. It is also difficult to transmit signals of this frequency range over long cable distances due to capacitive effects which degrade the signal quality. A further disadvantage is that spurious noise signals from other electrical sources may be picked up by the transmission line if such sources are anywhere on or near the line. A further disadvantage is that the control unit for each station is quite complicated and makes the cost of systems with even a low number of stations very high. The result of the above problems is usually the restriction of uses of such time sharing systems to smaller applications where the ease of wiring is a dominant factor.